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Junction-to-Case Thermal Response:
The magnitude of the MOSFET junction-to-case temperature
rise is the product of the MOSFET junction-to-case thermal
resistance (R ? J-C ), the sampled instantaneous power
dissipated in the MOSFET and the duration of the power
dissipation. The PI2211 internal model of R ? J-C includes both a
“pole” and a “zero” in the transfer function as an electrical
equivalent of the thermal resistance and storage components
of the MOSFET in the thermal model. The pole, Tau P , has a
slower response to the dissipated power than the zero Tau Z
does. Tau Z is internally calculated to be 1/8th of Tau P . R ? J-C
and Tau P are derived directly from the manufacturer’s
datasheet, and is further explained in the Recommended
Design Steps section.
The PI2211 junction-to-case thermal response is shown in
Figure 3; bounded by the internal allowed ranges of Tau P and
R ? J-C . The range of Tau P is 1ms to 15ms, the range of R ? J-C is
0.5°C/W to 7.5°C/W, and the minimum pulse width is 50us.
The "Power" term is the calculated power based on the value
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PI2211
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The calculated minimum R ? J-C is 1/8th of its nominal (steady-
state) value, meaning that the PI2211 will predict a higher
junction temperature rise than what the manufacturer's
curves would suggest for shorter pulse widths. This helps to
protect against transient hot-spotting in the MOSFET.
Case-to-Ambient Thermal Response:
The magnitude of the MOSFET case-to-ambient temperature
rise is a function of the MOSFET case-to-ambient thermal
resistance (R ? C-A ) and the instantaneous calculated power
dissipated in the MOSFET . The internal model has a “pole”
with a fixed thermal time constant (Tau) of 2 seconds and a
fixed R ? C-A of 60°C/W. A 2 second Tau will shut off the
MOSFET quickly at power levels that would typically require
100's of seconds to achieve an actual 60°C temperature rise,
protecting the MOSFET from thermal stress. 60°C/W is a
typical value of R ? C-A for packages with thermal tabs.
of R SOAS , which may be scaled from the true power. The
response of the PI2211 junction-to-case model is based on
the following equation:
Junction-to-Ambient Thermal Response:
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Using the ratios of Tau P to Tau Z , the equation simplifies to:
100
10
1
0.1
0.01
The over-all range of the junction-to-ambient thermal
impedance response of the PI2211 is shown in
Figure 4. This represents the entire adjustable range of the
dynamic R ? J-C and the fixed R ? C-A summed together.
1E-05
0.0001
0.001
0.01
0.1
1
10
Time
7.5°C/W, Tau = 1ms
.5°C/W, Tau = 15ms
Figure 4 - PI2211 J-A Thermal Impedance range of adjustment at 1W.
Picor Corporation · picorpower.com
PI2211
Rev 1.0, Page 10 of 26
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